1. Technical Field
The present invention relates, in general, to sonic well logging tools for radially evaluating cementing conditions around casing in cased wells and, in particular, to a method and system for circumferentially evaluating the placement and bonding strength of cement-sealing material disposed about the exterior surface of a tubular member within a wellbore in an acoustically fast formation.
2. Description of the Related Art
Prior-art sonic well logging tools have been used to evaluate cement bonding around casing within wellbores for many years. Typically, cement bonding is a term which has been used to describe a measure of an average compressive strength of cement disposed about a section of casing, which provides an indication of cement conditions within the wellbore, such as proper cure, mixture with borehole fluids and voids or channeling within a cement sheath. In general, cement-bonding measurements are used to provide an indication of cement placement about a well casing to determine whether the cement provides an adequate fluid seal to prevent fluids from flowing between portions of a wellbore.
Prior-art sonic well logging tools have been utilized to radially determine cement conditions within cased wellbores, from which the circumferential placement of cement about the exterior of a casing can be evaluated. For example, pulse-echo types of well logging tools have been used to transmit an initial sonic pulse radially outward from a sonic transducer to a spot on a casing wall, and the reflected sonic signal or echo then is received, utilizing a sonic transducer to evaluate the quality of the cement bond.
Examples of pulse-echo sonic tools are disclosed in U.S. Pat. Nos. 3,369,626; 4,255,798; 4,709,357; 3,369,626; 4,255,798; and 4,709,357, the contents of which hereby are incorporated herein by reference thereto.
A problem which exists with known techniques for measuring cementing conditions in cased wells is the presence of high-acoustic velocity or so-called "fast" formations. In such a situation, the velocity of the acoustic pulse in the high-acoustic velocity formation sometimes may be faster than the velocity of sound in the casing or pipe. Under these conditions, the return signal arrival through the pipe may be distorted by the arrival of acoustic signals travelling through the formation and, very often, this condition produces a combined arrival with a large amplitude which would otherwise indicate a poor bond or a total absence of a cement seal. Solutions to this problem have been proposed in U.S. Pat. No. 4,757,479 and 4,893,285 which disclose the addition of an additional receiver at a small distance from the transmitter. This solution is substantially ineffective in large diameter casings, such as 7 inches or greater, and/or when the cement layer is relatively thin--that is, one-inch thick or less.
It therefore should be apparent that a need exists for a method and system for accurately evaluating cement bond conditions between a tubular member and the wellbore in the presence of a high-acoustic velocity formation.